1. Field of Invention
The present invention relates to a power converting apparatus for system interconnection comprising one or more self commutated power converters or inverters (hereafter referred to as "self-commutated converters") for use in direct-current power transmission, frequency conversion, system interconnection, reactive power regulation, and the like.
More particularly the present invention pertains to a power converting apparatus including semiconductor valve structured series-connected or series-parallel-connected nonlatch type semiconductor devices (hereafter referred to as "nonlatch devices"), and is applicable to a large capacity equipment.
2. Description of the Related Art
Ordinarily, a self-commutated converter has a plurality of semiconductor valves connected in bridge connection. The respective semiconductor valves include a plurality of series-connected semiconductor devices, so as to produce a DC output, or an AC output of a desired frequency, by turning on and off the semiconductor devices in an alternate matter. The semiconductor devices can be provided as self-turnoff type semiconductor devices (hereafter referred to as "self turn-off devices") to take the place of thyristors. A self turn-off device is a semiconductor device capable of turning itself off by a gate signal or the like, such as a gate-turn-off thyristor (hereafter referred to as "GTO").
There are some advantages when self turn-off devices are incorporated in a converter as semiconductor devices. For example, the possibility of using GTO in converters has increased in large capacity equipment instead of conventional thyristor in converters.
When self turn-off devices are incorporated in a self-commutated converter which is applied in a general-purpose inverter or uninterruptable power supply system, the frequency of the pulse-duration modulation can be increased to a high-frequency on the order of 10 kHz to 20 kHz. Such apparatus using self turn-off devices can provide substantial improvement of its controlling characteristic.
However, in a case of a system interconnection including self-commutated converters, using GTOs as self turn-off devices, the switching frequency is restricted to about several hundred hertz. Therefore, it is desireable to increase the switching frequency of a self-commutated converter incorporated in a system interconnection to a high-frequency on the order of 10 kHz to 20 kHz, also.
Here, FIG. 7 shows an example of a convention of the structure of a system interconnection using a direct-current-alternating-current (hereafter referred to as "DC-AC") self-commutated converter.
A system interconnection comprises a self-commutated converter 3, a DC power supply 1 connected to a DC terminal of the self-commutated converter 3 through a DC reactor 2, and an AC power system 6 (hereafter referred to as "AC system") connected to an AC terminal of the self-commutated converter 3 through a connect reactor 4 and a transformer 5. The self-commutated converter 3 converts DC power supplied from the DC power system 1 into AC power and supplies the AC power to the AC system 6 through the series reactor 4 and the transformer 5.
However, the self-commutated converter 3 which uses GTOs as self turn-off devices in the system interconnection can have associated therewith problems as described next.
In a system so constructed, it is necessary to provide a snubber circuit which is connected in parallel to the GTO in order to protect the GTO from overvoltage at the time of the turn-off of the GTO. A snubber circuit includes a snubber capacitor and a discharge resistor which is connected in series with the snubber capacitor. The electric charge which is applied between both ends of the GTO charges the snubber capacitor at the time of the turn-off of GTO, and then is discharged through the discharge resistor while the GTO is conductive. Accordingly, most of the energy stored in a snubber capacitor is consumed as heat by the discharge resistor while the GTO is conductive. On the other hand, the handling to the energy has to exclusively rely on the snubber capacitor. In order to absorb the energy by the snubber capacitor, and the turn-off condition of the GTO, the capacitance of the snubber capacitor has to be increased. Also, in the case of a plurality of series-connected GTOs for a large capacitance converter, the capacity of the snubber capacitor has to be increased according to the voltage sharing. However, since the while energy stored in the snubber capacitor is consumed by the discharge resistor, the energy loss increases when the capacitance of the snubber capacitor is increased. As a converter capacity increases, the snubber circuit loss becomes serious. In other words, the efficiency of the self-commutated converter is lowered. This snubber circuit loss becomes very large compared with other circuit losses. Therefore, when GTOs are incorporated in a self-commutated converter, it is difficult to provide for and self-commutated converter for use in a large capacity converter, in view of the undesirable losses caused by the snubber circuit.
Further, immediately after GTO conduction is cut off, the voltage applied between the GTO's anode and cathode increases gradually, and a spike voltage Vsp is generated, as shown in FIG. 8.
The spike voltage Vsp increases with the increase in the anode current. When the spike voltage Vsp increases above a certain level, the GTO may break down. Therefore, it is necessary that the spike voltage Vsp be decreased. Generally, the larger the capacity of the snubber capacitor and the smaller the series inductance of the wiring of the circuit, the more the spike voltage Vsp is decreased. Accordingly, it is necessary that the capacitance of the snubber capacitor is increased. Also, to decrease the spike voltage Vsp, it has been proposed to set a minimum on-time Td, in order to dissipate the electric charge in the snubber capacitor during the on-operation of the GTO. That is, the larger the capacity of the snubber capacitor, the longer the minimum on-time Td becomes. When there is a such limitation in minimum on-time Td, there are some problems as descried next. When switching frequency increase, 1) DC voltage due to increase in order to obtain necessary AC voltage, therefore the voltage harmonic increases 2) and, control response becomes worse so that control impossible time be increased.
In the case of a plurality of series-connected GTOs, if the capacity of the snubber capacitor is decreased, the division of voltage among the respective GTOs (as self turn-off devices) is not uniform. According to the variation of voltage sharing, an overvoltage may be applied to a particular GTO.
As a result, in the case of a self-commutated converter using self turn-off devices for system interconnection, the energy loss increases when the capacitance of the snubber capacitor is increased. On the other hand, if the capacitance of the snubber capacitor is not increased in order to prevent reduction of the efficiency, the voltage sharing applies to each of the self turn-off devices which are connected in series is not uniform. As a result, an overvoltage may be applied to a particular self turn-off device, and the magnitude of the spike voltage Vsp cannot be decreased.